My problem is that why would God keep us here for another 1000 years after Christ returns on the last day. When Christ returns He will destroy the enemy by the spirit (word) of His mouth and the brightness of His coming, 2 Thessalonians 2:1-12; Rev 19:11-21.
To save as many as He can.
After Christ slays the Beast and his army at Armageddon and casts him and the false prophet into Gehenna/Lake of Fire, He judges the survivors on earth:
31 "When the Son of Man comes in His glory, and all the holy angels with Him, then He will sit on the throne of His glory.
32 "All the nations will be gathered before Him, and He will separate them one from another, as a shepherd divides his sheep from the goats.
33 "And He will set the sheep on His right hand, but the goats on the left. (Matt. 25:31-33 NKJ)
This judgment concerns all on earth who heard the Gospel of Christ preached, but did not become believers. You can't be a "sheep" or a "goat" until you have had the gospel preached to you.
Those sheep who helped Christians while they were being persecuted by the Antichrist, enter into Christ's eternal kingdom, joining us Kings and priests.
The goats who did nothing for Christ's brethren are cast into the lake of fire to join the Antichrist and false prophet:
34 "Then the King will say to those on His right hand,`Come, you blessed of My Father, inherit the kingdom prepared for you from the foundation of the world:
35 `for I was hungry and you gave Me food; I was thirsty and you gave Me drink; I was a stranger and you took Me in;
36 `I was naked and you clothed Me; I was sick and you visited Me; I was in prison and you came to Me.'
37 "Then the righteous will answer Him, saying,`Lord, when did we see You hungry and feed You, or thirsty and give You drink?
38 `When did we see You a stranger and take You in, or naked and clothe You?
39 `Or when did we see You sick, or in prison, and come to You?'
40 "And the King will answer and say to them,
`Assuredly, I say to you, inasmuch as you did it to one of the least of these My brethren, you did it to Me.'
41 "Then He will also say to those on the left hand,`Depart from Me, you cursed, into the everlasting fire prepared for the devil and his angels:
42 `for I was hungry and you gave Me no food; I was thirsty and you gave Me no drink;
43 `I was a stranger and you did not take Me in, naked and you did not clothe Me, sick and in prison and you did not visit Me.'
44 "Then they also will answer Him, saying,`Lord, when did we see You hungry or thirsty or a stranger or naked or sick or in prison, and did not minister to You?'
45 "Then He will answer them, saying,`Assuredly, I say to you, inasmuch as you did not do it to one of the least of these, you did not do it to Me.'
46 "And these will go away into everlasting punishment, but the righteous into eternal life." (Matt. 25:34-46 NKJ)
The rest of the earth who had no connections to "Christ's brethren" (which implies they had not heard the gospel preached sufficiently to incur liability for not believing it), enter the 1000 year kingdom of Christ. They are allowed to marry and populate the earth resulting in billions more of humanity to enter God's Kingdom.
If anyone begins sinning during Christ's millennial kingdom he is cast into the lake of fire, "accursed":
18 But be glad and rejoice forever in what I create; For behold, I create Jerusalem as a rejoicing, And her people a joy.
19 I will rejoice in Jerusalem, And joy in My people; The voice of weeping shall no longer be heard in her, Nor the voice of crying.
20 "No more shall an infant from there live but a few days, Nor an old man who has not fulfilled his days; For the child shall die one hundred years old,
But the sinner being one hundred years old shall be accursed.
21 They shall build houses and inhabit them; They shall plant vineyards and eat their fruit.
22 They shall not build and another inhabit; They shall not plant and another eat; For as the days of a tree, so shall be the days of My people, And My elect shall long enjoy the work of their hands.
23 They shall not labor in vain, Nor bring forth children for trouble; For they shall be the descendants of the blessed of the LORD, And their offspring with them.
24 "It shall come to pass That before they call, I will answer; And while they are still speaking, I will hear.
25 The wolf and the lamb shall feed together, The lion shall eat straw like the ox, And dust shall be the serpent's food. They shall not hurt nor destroy in all My holy mountain," Says the LORD. (Isa. 65:18-25 NKJ)
Then Satan is let out of his prison to again tempt mankind to rebel, and many will because they hate light, holiness, and crave evil. These will attack Jerusalem and God will destroy them:
7 Now when the thousand years have expired, Satan will be released from his prison
8 and will go out to deceive the nations which are in the four corners of the earth, Gog and Magog, to gather them together to battle, whose number is as the sand of the sea.
9 They went up on the breadth of the earth and surrounded the camp of the saints and the beloved city. And fire came down from God out of heaven and devoured them.
10 The devil, who deceived them, was cast into the lake of fire and brimstone where the beast and the false prophet are. And they will be tormented day and night forever and ever. (Rev. 20:7-10 NKJ)
So billions more will enter God's Kingdom because of the Millennial reign of Christ.
Christians who are raptured during Christ's parousia and all the martyrs take part in the first resurrection and join us Christians who died before Christ's return, to serve in Christ's millennial kingdom as kings and priests and we will convert many:
4 And I saw thrones, and they sat on them, and judgment was committed to them. Then I saw the souls of those who had been beheaded for their witness to Jesus and for the word of God, who had not worshiped the beast or his image, and had not received his mark on their foreheads or on their hands. And they lived and reigned with Christ for a thousand years.
5 But the rest of the dead did not live again until the thousand years were finished. This is the first resurrection.
6 Blessed and holy is he who has part in the first resurrection. Over such the second death has no power, but they shall be priests of God and of Christ, and shall reign with Him a thousand years. (Rev. 20:4-6 NKJ)
So AFTER the 1000 year Kingdom the Great White Throne Judgment of all who ever lived, occurs:
11 Then I saw a great white throne and Him who sat on it, from whose face the earth and the heaven fled away. And there was found no place for them.
12 And I saw the dead, small and great, standing before God, and books were opened. And another book was opened, which is the Book of Life. And the dead were judged according to their works, by the things which were written in the books.
13 The sea gave up the dead who were in it, and Death and Hades delivered up the dead who were in them. And they were judged, each one according to his works.
14 Then Death and Hades were cast into the lake of fire. This is the second death.
15 And anyone not found written in the Book of Life was cast into the lake of fire. (Rev. 20:11-15 NKJ)
Billions more are saved because only those who refused to repent in hell and believe in Jesus are not found written in the book of life, and so cast into the lake of fire.
Only then, after God has completely separated the wheat from the chaff, will the present earth and heavens pass away in a fervent heat, and the New Heaven and New Earth are created.
Then New Jerusalem comes down from heaven and God dwells with man on Earth:
1 Now I saw a new heaven and a new earth, for the first heaven and the first earth had passed away. Also there was no more sea.
2 Then I, John, saw the holy city, New Jerusalem, coming down out of heaven from God, prepared as a bride adorned for her husband.
3 And I heard a loud voice from heaven saying, "Behold, the tabernacle of God is with men, and He will dwell with them, and they shall be His people. God Himself will be with them and be their God.
4 "And God will wipe away every tear from their eyes; there shall be no more death, nor sorrow, nor crying. There shall be no more pain, for the former things have passed away."
5 Then He who sat on the throne said, "Behold, I make all things new." And He said to me, "Write, for these words are true and faithful."
(Rev. 21:1-5 NKJ)